Marine structures



Dec. 23, 1969 w. M MARTINOVICH MARINE STRUCTURES Filed Oct. 12, 1967FIG.

FIG. 2

FIG. 3

INVENTOR:

WILLIAM M. MARTINOVICH FIG. 4

HIS AGENT US. Cl. 61--46 United States Patent 3,485,050 MARINESTRUCTURES William M. Martinovich, San Francisco, Calif., asslgnor toShell Oil Company, New York, N.Y., a corporation of Delaware and Earland Wright, San Francisco, Calif a corporation of California, as tenentsin common Filed Oct. 12, 1967, Ser. No. 674,909 Int. Cl. E0211 17/00;E04h 12/10; E04c 3/32 9 Claims ABSTRACT OF THE DISCLOSURE A marinestructure or offshore platform having large diameter legs andinterconnecting cross-bracing members of smaller diameter in which thecross-bracing members are provided with bifurcated ends formingspread-apart arms that are connected to the edges of the legs. Theresultant structure smoothly transfers the loading from thecross-bracing to the legs while minimizing stress concentrations whichtend to cause the legs to collapse or radially deform.

Background of the invention This invention relates to marine structuresand more particularly to offshore platforms having a working deckarranged above the surface of the water supported by legs resting on ordriven parrtially into the floor of the body of water.

Conventional platforms are generally constructed with a plurality oflegs that are interconnected below the water surface by cross-bracingmembers to give the paltform more rigidity. The cross-bracing membersusually consist of tubular members arranged either horizontally ordiagonally between the legs when viewed in elevation. In addition, thecross-bracing members are generally of a smaller diameter than the legs.Even when single leg structures are employed to support a platform, theleg is generally provided with sloping supports or braces extendingbetween the leg and the floor of the body of water.

Regardless of the type of structure or the number of legs employed, atleast one end of the brace must be attached to the platform leg. In thecase of a tubular cross-brace being attached to a tubular leg, theconventional practice is to shape the end of the cross-brace to conformto the curved surface of the leg and then attach the brace to the leg bysuitable means such as welding. The resultant structure is the same asthe geometrical intersection of two cylinders whose axes are in the sameplane. In the case of a cross-brace attached to a leg of a right angle,the structure is similar to that of a chair leg and rung.

When the cross-bracing member has a smaller diameter thanthe platformleg, as is typically the case, a problem arises in connecting themembers in a manner to Withstand operational loading without failure.Since the crossbrace is smaller, the load carried by it is transmittedto only a small portion of the leg. The natural tendency of the leg,being a tubular member, is to resist the loading imposed by thecross-brace at the edges of the leg rather than directly in the center.Thus, a convevntional cross-brace which intersects a platform leg in thecenter will tend to collapse the leg or cause large radial deformationof the leg. Merely increasing the size of the leg to withstand theloading is unsatisfactory since the increase adds appreciably to theWeight of the structure which makes it more difficult to handle.Furthermore, the cost of the structure Would be considerably increased.

The platform leg cannot be adequately reinforced or 3,485,050 PatentedDec. 23, 1969 stiffened internally due to space limitations since thelegs contain piles, drilling tubes, diver access tubes and the likewhich consume the majority of the interior of the tubular leg. Gussetplates and the like have been employed on the exterior of the leg sothat. the load transfer takes place from the small diameter cross-braceto the gusset plate and then to the large diameter leg, However, thisindirect transfer causes stress concentrations which can drasticallyreduce the fatigue life of the joint and structure.

The aforementioned problems are particularly aggravated in offshoreplatforms since these structures are always being dynamically loaded bywave, current, ice and wind forces which produce many cycles of loading.Accordingly, there is a need for structural tubular joint for use withan offshore platform which is extremely efficient and relatively free ofstress concentrations in order to provide a safe structure.

Summary In view of the shortcomings of the prior art as discussed above,it is a primary object of this invention to provide an improved platformstructure capable of withstanding the bending and torsional moments andloading produced in a marine structure under operating conditions.

Another object of this invention is to provide a tubular joint for usein offshore platforms that transfers load in a manner that minimizesstress concentrations.

It is another object of this invention to provide an improvedcross-bracing member for use on offshore platforms.

It is still another object of the invention to provide a small diametertubular cross-bracing member having a bifurcated end which is attachedto a large diameter tubular platform leg.

It is a further object of the invention to provide a tubularcross-bracing member for a platform leg in which the bifurcated arms areformed of a unitary piece with the main body of the cross-bracingmember.

It is a still further object to provide a cross-bracing member in whichthe bifurcated arms are formed of separate pieces rigidly attached tothe cross-bracing member and the leg.

In accordance with these objects and others which may become apparent onfurther reading, the subject invention provides an improved platformstructure having superior strength over prior art structures and iscapable of Withstanding the operating conditions encountered offshore.More specifically, the invention provides an offshore structure in whichthe cross-bracing members interconmeeting the legs of the structure areformed with special ends which distribute the loading between the legand the brace over a wider area resulting in a more rigid joint. Morespecifically, the special ends are in the form of spread-apart armswhich engage the platform leg near the edges of the leg so that thetransfer of load from the small diameter cross-brace to the largediameter platform leg is a smooth transition, thereby reducing stressconcentrations. This results in a joint having va long fatigue life forthe cyclic or continuous loading encountered in an offshore environment.

Brief description of the drawings FIGURE 1 is an elevational view of atypical offshore platform incorporating the subject invention;

FIGURE 2 is a cross-sectional view of a leg of the platform taken alongthe line 2-2 of FIGURE 1.

FIGURE 3 is a cross-sectional view of a leg of the platform taken alongthe same line but showing an alternative construciton; and,

FIGURE 4 is a cross-sectional view of a cross-brace taken along the line44 of FIGURE 3.

Description of the preferred embodiments The subject invention may besuitably employed on several different types of offshore installations.One such installation shown in FIGURE 1, is a so-called platformpositioned in a body of water and used for drilling oil or gas wells inthe water floor 11. The platform includes a plurality of legs 12(generally four) supported on or partially driven into the floor 11 andadapted to support a working deck 13 from which the drilling operationsmay be performed. Located on the deck are the normal complements of adrilling operation including crew quarters 14, derrick 15 and the like.The legs 12 are of a tubular construction and are interconnected bytubular bracing members including a horizontal bracing member 16 anddiagonal bracing members 17. In the case of a four-legged structure thebracing members and legs form a generally box-like or rectangularstructure.

The cross-sectional view of one of the legs 12 shown in FIGURE 2 alsoshows the underside of the crossbracing member 16 and is directed towardone of the embodiments of the subject invention. As seen in FIG- URE 2,the cross-brace 16 has a diameter smaller than the platform leg 12 andis connected to the leg by means of a pair of spaced arms 18 and 19.Each arm is in the form of a half-round cylinder made from a tubularpiece of approximately the same diameter as the cross-brace 16 which hasbeen split longitudinally. The arms converge at the cross-bracing member16 where they are attached thereto by suitable means, for example,welding, so that the outer surface of the arms is substantially acontinuation of the outer surface of the cross-brace 16. The arms arepreferably shaped to conform with the outer surface of the leg 12 wherethey are suitably attached to the leg 12, e.g., by means of welding. Thespread between the arms 18 and 19 is preferably designed so that at thewidest point of divergence the outer edges of the arms are approximatelyequal to the diameter of the leg 12. The spread of the arms may, ofcourse, be less but in no event should the spread be greater than thediameter of the leg.

The V-shaped space formed between the arms 18 and 19 is preferablyclosed by means of a flat plate 20 welded to the upper and lower sidesof the arms and also to the leg 12. The plates 20 serve to addadditional rigidity to the structure and also prevent sea Water fromentering the interior of the cross-brace 16.

Another embodiment of the invention is shown in FIGURE 3. In thisarrangement, cross-bracing member 16a corresponds to bracing member 16of the FIGURE 2 embodiment and includes arms 21 and 22 corresponding toarms 18 and 19. However, the bracing member 16a and arms 21 and 22 areconstructed from the same piece rather than being constructed ofseparate parts as in the embodiment shown in FIGURE 2. The end ofbracing member 16a is bifurcated to form arms 21 and 22 by splitting thetubular brace 16a longitudinally and spreading the ends apart. Arms 21and 22 are cut to conform to the leg 12 in the same manner as discussedabove with reference to arms 18 and 19 and are welded to the leg 12.Suitable plates 23 may be Welded to the upper and lower sides of thebracing member 16a and also to the leg 12 to close the opening formed bythe two legs 21 and 22. These plates are similar in design and functionto the plates 20 disclosed with reference to the embodiment of FIGURE 3.

The composite structure of both the embodiment of FIGURE 2 and theembodiment of FIGURE 3 is the same. In each instance, a bracing memberis bifurcated at the point of juncture with the platform leg so that thebrace is attached to the platform by means of spaced arms rather thanbeing attached directly to the platform.

The interior of the cross-bracing member in either embodiment may beprovided with suitable reinforcing at the point of convergence of thearms. For example, 7

as shown in FIGURES 3 and 4 a generally cylindrical, longitudinallyextending flange 24 is positioned concentrically inside the brace 16a bymeans of an annular fiat web 25 attached to the flange and the brace16a. Longitudinally extending gusset plates 26 may be suitably weldedinto place at the top and bottom of the flange 24 as shown in FIGURE 4and extend between the web 25 and the leg 12 as shown in FIGURE 3.Flange 24 and web 25 serve to reinforce the cross-brace 16a. Gussetplates 26 are attached to the leg 12 to add further reinforcement andalso serve as additional support for the plates 23 which close theopenings formed by the arms.

Although not shown, the embodiment of FIGURE 2 may employ similarreinforcing means. In addition, the reinforcing means may be omitted orother forms of spirit of the invention.

It will be readily apparent that the construction shown in FIGURE 2 andthe construction shown in FIGURE 3 may be readily interchanged with eachother. Therefore, either embodiment or a combination of both may beadapted to this cross-bracing used when constructing a platform. Inaddition, the subject invention is equally suited for diagonal bracingmembers such as the members 17 shown in FIGURE 1. It will also bereadily apparent that although the invention is shown as applied to aplatform, it is equally suited for other marine structures where bracingis employed; for example, docks, bridges, dolphins and the like.

In operation, that is, when the platform is positioned in a body ofwater and subject to the forces of wind, ice, waves, current and thelike, the subject invention provides greatly improved cross-brace-to-legjoint which transfers the load from the cross-brace out to the edges ofthe platform leg while minimizing or eliminating harmful stressconcentrations that could produce structural failure. The bifurcatedbracing member described also possesses a greater resistance to twistingor torsion in the leg as well as lateral bending of the bracing memberdue to the spaced arms which distribute the loading over a wider area onthe platform leg.

I claim as my invention:

1. In a marine structure of the type having leg means supported by thefloor of a body of water, a platform carried by said leg means andstructural bracing attached to said leg means, an improved structurecomprising:

at least one substantially vertical tubular leg:

an elongated, tubular, structural, bracing member having a smallerdiameter than said leg; and,

at least one end of said bracing member terminating in a pair ofdivergent spaced tubular arms partially surrounding and conforming tosaid leg and connected directly thereto;

said arms having a spread greater than the diameter of said bracingmember and no greater than the diameter of said leg to thereby form asubstantially V-shaped opening between said arms when viewed vertically.

2. A structure as defined in claim 1 including plate means welded tosaid arms and said leg to close said V-shaped opening.

3. A structure as defined in claim 2 wherein said arms and said bracingmember comprise a unitary element.

4. A structure as defined in claim 2 wherein said arms comprise separateelements welded to said bracing member.

5. A structure as defined in claim 2 wherein both ends of said bracingmember terminate ina pair of spaced arms.

6. A structure as defined in claim 5 wherein said arms comprisehalf-round cylindrical members having substantially the same diameter assaid bracing member and welded to said bracing member so that the outersurface of said arms is a continuation of the outer surface of saidbracing member.

7. A structure as defined in claim 2 including reinforcing means carriedinternally of said arms and said bracing member.

8. A structure as defined in claim 7 wherein said reinforcing meanscomprise:

concentric, longitudinally extending flange means positioned at thepoint said arms intersect said bracing member;

annular web means carried between said flange means and said bracingmember and secured to said flange means and said bracing member; and,

gusset plate means secured to said flange and Web means and extending tosaid leg.

9. A structure as defined in claim 2 wherein said leg means comprisefour legs and said structural bracing comprises cross-bracesinter-connecting said legs, said composite structure having a generallyclosed box-like configuration.

References Cited UNITED STATES PATEN'L S 692,681 2/ 1902 Leonard 526971,822,389 9/ 1931 Blakely. 2,189,201 2/ 1940 Flader. 3,253,417 5/ 1966Manning 61-465 3,021,159 2/1962 Back 287189.36 X

JACOB SHAPIRO, Primary Examiner US. Cl. X.R.

